[evlatests] Master LO frequency references and the 1.7ms discrepancy

[Vivek Dhawan]

A short VLA-PieTown fringe test was done last night.  I can confirm that the
1.7ms is gone, the VLA was at ~2.5usec offset from UT.

There is however a fierce rate of drift, ~5Hz at C band or  10^-9, so about
10 times more than Keith's 9.8us/day

There are also various warts on the phase - and a lower fringe amplitude,
though I have not yet done a careful comparison - from the short-term
(seconds) instability of the rubidium.


On Thu, April 18, 2013 07:30, Keith Morris wrote:
| As it stands now, the L356 firmware version that is installed at the site
| accurately measures the drift, but not the offset, between the GPS and the
| synthesized pulse.  We have been testing a new firmware version in the lab
| in which we attempt to address the offset problem, but it is not yet ready
| to deploy.
|
|
|
|
|> It seems to me that the monitor point design must be changed to
|> measure the lag between the L356 and the GPS, so this doesn't happen
|> again.
|>
|>> We located the source of the 1.7 millisecond timing discrepancy.  The
|>> GPS
|>> 1PPS signal and the 1PPS signal generated in the L356 were
|>> mis-synchronized by exactly this amount, with the L356 lagging in
|>> time.
|>> This is due either to the L356 having been synchronized to the wrong
|>> mark,
|>> or not having been synchronized at all.  We resynchronized the two
|>> pulses
|>> this morning at 11am.
|>>
|>> Then, at 11:40am, we switched the master frequency reference from
|>> Hydrogen
|>> Maser to Rubidium, as planned.  The PPS that is synthesized from the
|>> Rubidium 5MHz is drifting relative to the GPS at a rate of about 9.8
|>> microseconds/day.
|>>
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|>>
|>
|>
|
|
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